1. Field of the Invention
This invention relates to the field of compatibilization technology. In particular, this invention relates to the use of ethylene/methacrylate/acrylic acid terpolymers as compatibilizers for dissimilar elastomer blends.
2. Description of the Related Art
A considerable amount of research has been made over the last several years with a view to obtaining new polymeric materials with enhanced specific attributes for specific applications or a better combination of different attributes. Much attention is currently being devoted to the simplest route for combining outstanding properties of different existing polymers, that is, formation of polymer blends. Although increasing numbers of miscible blends are reported in the literature [D. R. Paul et. al., J. Macromol. Sci., Rev. Macromol. Chem., C-18:109 (1980)], most polymers are nonetheless immiscible thus leading to heterophase polymer blends. In general, "compatibility (miscibility) is the exception, immiscibility is the rule" (Dobry and Boyer-Kawenski, J. Polymer Science, 1947).
There are two widely useful types of elastomer blends: single phase and two phase blends. The single phase blend is miscible. The term miscibility does not imply ideal molecular mixing but suggests that the level of molecular mixing is adequate to yield macroscopic properties expected of a single-phase material.
The formation of two-phase elastomer blend is not necessarily an unfavorable event since many useful properties, characteristic of a single phase, may be preserved in the blend composition while other properties may be averaged according to the blend composition. Proper control of overall elastomer blend morphology and good adhesion between the phases are in any case required in order to achieve good mechanical properties. The elastomer blend components that resist gross phase segregation and/or give desirable blend properties are frequently said to have a degree of "compatibility" even though in a thermodynamic sense they are not "miscible". It should be emphasized that "compatibility" and "miscibility" are two different terms. Compatibilization means the absence of separation or stratification of the components of the polymeric alloy during the expected useful lifetime of the product (Gaylord, N. G., in "Copolymers, Polyblends and Composites" Advances in Chemistry Series 142, American Chemical Society: Washington, D.C., 1975, p. 76). "Technological compatibilization" according to Coran and coworkers [Rubber Chem. Technol., 56, 1045 (1983)] is "the result of a process or technique for improving ultimate properties by making polymers in a blend less incompatible; it is not the application of a technique which induces "thermodynamic compatibility", which would cause the polymers to exist in a single molecularly blended homogeneous phase".
It is well established that the presence of certain polymeric species, usually block or graft copolymers with the right structure, can indeed result in compatibilization of an immiscible elastomer blend because of their ability to alter the interfacial situation. Such, species as a consequence, are often referred to as "compatibilizers" or "interfacial agents" which is analogous to the term "solubilization used in the colloid field to describe the effect surfactants have on the ability to mix oil and water (McBain et. al., "Solubilization and Related Phenomena", Academic Press, New York, 1955). Such "compatibilizers" can be either preformed and added to the binary blend or formed "in situ" during the blending process.
The role of the compatibilizer in an elastomer blend is manifold: (1) reduce the interfacial energy between the phases, (2) permit a finer dispersion during mixing, (3) provide a measure of stability against gross segregation, and (4) result in improved interfacial adhesion (G. E. Molau, in "Block Copolymers", Ed by S. L. Agarwal, Plenum, N.Y., 1970, p. 79).
Two elastomers form a compatible mixture when they have at least one of the following characteristics:
Segmental structural identity. For example, a graft or block copolymer of butadiene and styrene is compatible with either polybutadiene or polystyrene. PA1 Miscibility or partial miscibility with each other. Solubility parameter ( ) difference &lt; 1, generally &lt;0.2 units. For example, poly (vinyl chloride), PVC, poly (ethylacrylate), PEA, poly (methylacrylate), PMMA, have solubility parameters in the 9.4-9.5 range and form compatible mixtures. Although, the structure of nitrile rubber, NBR is entirely different from those of PVC, PMMA, PEA, it has a similar solubility parameter 9.5 and is compatible with these three polymers. PA1 Functional groups capable of generating covalent, ionic, donor-acceptor or hydrogen bonds between the polymers.
Compatibilization of dissimilar elastomer blends is an area of active interest from both technological and scientific points of view. Many of the synthetic and natural elastomers have good properties that when combined with other rubbers of similar or complementary properties may yield desirable traits in the products.
Neoprene or polychloroprene rubber (CR) has been the material of choice in most power transmission belts, due to its unique combination of properties: ozone resistance, oil resistance, toughness, dynamic flex life, good adhesion to other materials and heat resistance up to 100.degree. C. In the past, CR belts have kept pace with the needs of the automotive industry, but recently there is a need for new materials for more demanding applications. First of all, CR belts are encountering greater heat duress in service due to increasing underhood temperatures (up to 150.degree. C.). Secondly, to meet automotive industry's longer warranty periods ("100,000 mile target"), the CR belts must have a lower failure rate with high mean life, even when high temperatures are not encountered. To meet these emerging needs, improvements in heat, ozone, and cut growth resistance of neoprene belts are desirable. The above requirement for neoprene belts could be satisfied by blending with polyolefin elastomers such as ethylene/propylene rubber (EPR) or ethylene/propylene/diene terpolymer (EPDM) which have better heat/ozone and cut growth resistance. As such, however, these neoprene/EPR or EPDM blends are incompatible.
Nitrile rubber (NBR) is used in automobiles because of its resistance to fuels, a variety of oils and other fluids over a wide range of temperatures. However, nitrile rubber, as such cannot be used in specific applications requiring high heat and ozone resistance. The poor ozone resistance and heat ageing properties of NBR (which is a random copolymer of acrylonitrile and butadiene) are believed to be the result of unsaturation in the backbone of the polymer which permits scission of the polymer chain to occur under certain adverse conditions.
EPDM rubber, on the other hand, has good heat ageing and ozone resistance because, its unsaturation sites are in side chains which render the polymer generally immune to scission of the backbone chain. However, these EPR or EPDM rubbers have poor oil resistance even in the cured state. It is desirable to achieve the best properties of both NBR and EPDM rubber, i.e. improved heat, ozone and oil resistance by blending the said rubbers synergistically. Such NBR/EPDM blends could find numerous applications in the automobile industry. As such, however, these NBR/EPDM blends are incompatible, because of the polarity difference between the blend components.
It is known in the art that the resistance of cured unsaturated elastomers such as polybutadiene or polyisoprene to chemical attack from ozone and oxygen can be enhanced by forming a blend thereof with minor amounts of an ethylene/propylene/diene terpolymer and co-vulcanizing the blend. This development takes advantage of the inherent resistance of the olefin/diene terpolymer to chemical attack and imparts this property into co-vulcanized blend.
However, the use of olefin/diene terpolymers in blends with other elastomers is often limited to those other elastomers which have a mutual compatibility and comparable cure rate behavior with respect to the olefin/diene terpolymer. Thus, whereas highly unsaturated elastomers such as polybutadiene or polyisoprene may be, in some cases reasonably compatible with olefin/diene elastomers and may be readily co-vulcanized because of the high availability of sites of ethylenic unsaturation, other elastomers such as polychloroprene, butadiene/acrylonitrile copolymers (nitrile) and like materials containing polar groups along the chain and/or a relatively low degree of ethylenic unsaturation are not so readily co-vulcanized. In the case of blends with these latter elastomers, chemical resistance may be improved due to the influence of the olefin/diene terpolymer, but often at the expense of a lowering of physical properties such as tensile strength, elongation, modulus and/or abrasion resistance of the co-vulcanizate as compared with the cured elastomer itself.
Furthermore, many rubber compounds contain carbon black as a filler to increase strength, rigidity and other factors. Thus, a rubber blend must also be able to incorporate carbon black to be of use in the automotive industry. However, for blends of dissimilar elastomers, problems can arise in achieving optimum carbon black distribution between the microphases of the final product. In blends of elastomers that differ significantly in terms of unsaturation or viscosity, carbon black tends to locate preferentially in the higher unsaturation or lower viscosity phase.
Carbon black aggregates may also transfer from one elastomer to another during mixing if they are contained in a polymer of low unsaturation, or in a masterbatch with high extender oil content and relatively low heat history. Polarity is also a factor controlling carbon black migration in elastomer blends. Carbon black has been shown to transfer or migrate between natural rubber (NR, polyisoprene), and polychloroprene (CR). However, it was observed that most of the carbon black in such a NR/CR blend remained at the interface--Marsh, P. A. Rubber Chem. and Tech. 41,344 (1968). Other research has shown undesirable displacement of the carbon black by more polar elastomers, Craig P. and Fowler R. B., Rubber World, 146(6), 79, (1962).
Thus, it would be of great importance to the art if a compatibilizer for dissimilar rubber blends such as CR/EPDM, NBR/EPDM could be found. It would be of further advantage to the art if this compatibilizer also caused delocalized dispersion of the carbon black in the above dissimilar rubber blends.
Use of an ethylene/acrylate/acrylic acid terpolymer as a compatibilizer for rubber blends is not known in the technology. Of tangential interest may be U.S. Pat. No. 4,607,074 to Hazelton where a cured rubber, an uncured rubber and a polyolefin are blended. The polyolefin is taught to be a copolymer of ethylene and unsaturated esters of C.sub.1 to C.sub.4 monocarboxylic acids. Hazelton does not disclose a terpolymer of ethylene/acrylate/acrylic acid.
In addition, U.S. Pat. No. 4,307,204 to DuPont discloses an expandable, curable elastomeric sponge composition based on ethylene/propylene/diene terpolymer (EPDM) elastomer or polychloroprene elastomer, which composition further contains a minor amount of an ionomer resin which is an ethylene polymer or copolymer containing at least about 50 mole percent acid functional groups, which groups are at least 50% neutralized by metal ions. These acid-modified ethylene polymers, which may also include acid-modified EPDM terpolymers, are disclosed to improve the balance of curing and expanding properties of the polymer composition when used to prepare cured expanded materials.
None of the aforementioned disclosures addresses the development of a compatibilized polychloroprene/EPDM or EPR or nitrile rubber/EPDM or EPR blends which not only exhibit improved resistance to ozone or oxygen attack and improved heat stability, but also exhibit a retention and in some cases improvement of important physical properties such as tensile strength, elongation, modulus and resistance to abrasion. Also, the said references do not address the issue of carbon black distribution in a binary, dissimilar elastomer blends and ways to improve the distribution of carbon black in both phases of such blends.